The present invention relates generally to fuel mixing and combustion in a fluid stream. In particular, this invention relates to apparatus for injecting fuel into a high speed fluid stream to increase mixing and combustion efficiency of the fuel.
In many arts it is necessary to inject a fuel into a fluid, for example, air, in a manner that will promote mixing and combustion of the fuel in a commercially acceptable period of time and space. If the air stream is moving relative to the point of injection of the fuel, a persistent problem occurs in that the fuel may not properly penetrate the air stream a distance sufficient for the fuel to mix with the air by the time the air stream leaves the combustion chamber. In supersonic combustion ramjets (scramjets) where the fuel must be injected into a supersonic airstream, the problem becomes critical in that mixing and combustion of the fuel and air must occur extremely rapidly to achieve efficient operation, i.e., before the fuel leaves the combustion chamber.
A second problem with fuel combustion in jet propulsion devices, and in particular scramjets, is that operating at such high velocities makes the engine extremely sensitive to component efficiency. For example, if the inlet air stream loses an additional 1% of the available kinetic energy (kinetic energy efficiency goes from 98% to 97%) it is very likely that the engine would cease to produce useful thrust. Any component that introduces losses in the combustion process can quickly degrade the ability of the engine to produce thrust. Therefore, the manner of introducing the fuel into the air stream is extremely important.
Fuel injection schemes have been developed which address these problems, but fall short of producing adequate results. The most obvious method to get more fuel into the air stream was to simply pump much greater amounts of pressurized fuel into the air stream from the side in the manner of a large orifice. However, air/fuel mixing is not well served by having a few large injectors because the result is a large over fueled region surrounded by underfueled air.
If limited to injecting fuel from a wall of the combustor to penetrate the air stream, one solution is to have injectors on all sides so that injector penetrates far enough, say 1/4 of the duct width. Once the fuel is out in the stream, the fuel and air mix so all of the surrounding air gets fuel. The manner of mixing depends on the space between the fuel plumes. The term "gap" is commonly used in the art to mean the distance between the fuel injection plumes. It is frequently used to describe the relative distance the fuel travels before it mixes adequately with the air (burning is substantially instantaneous after mixing). However, has been found that using conventional sonic injectors separated by gap of "G", the fuel enters the supersonic air stream and turns parallel to the air stream, and a distance of perhaps 60 times G is required to achieve significant mixing. This leads to the desire for more closely spaced injectors to reduce the required combustor length. However, adding more injector sites (closer together) would result in extra fuel being injected which reduces engine efficiency due to the incomplete combustion. Reducing the fuel flow to the desired level (by reducing feed pressure or injector orifice size) reduces the fuel penetration and leaves air near the center of the combustor without sufficient fuel for combustion.
One object of the present invention is to provide the engine designer with fuel injectors that penetrate relatively better than prior art fuel injectors so that the designer can use a large number of injectors (reduced mixing gap) without wasting fuel or starving the center of the duct. FIG. 1 illustrates the desired result, comparing fuel plumes from conventional sonic injectors (FIGS. 1a and 1b) with fuel plumes from injectors according to the present invention (FIGS. 1c and 1d). In all four views of FIG. 1, "I" represents the respective injector. FIGS. 1a and 1b show that with seven sonic injectors producing fuel plumes separated by a mixing gap G requires a combustor length roughly equal to 60 G, while the improved injector with the same penetration requirement provides more injection sites (here 15, for example) producing a mixing gap of g, requiring a reduced combustor length of 60 g, where g&lt;G.
One primary improvement in the presently claimed fuel injectors is the increased relative penetration of the fuel jet. When fuel is injected into a cross flowing air stream, an aerodynamic interaction occurs that deflects the fuel plume until it becomes parallel with the air stream. The point where the fuel plume becomes parallel to the duct wall is the point of maximum penetration. The distance the fuel jet penetrates is determined by the trajectory of the fuel jet. The trajectory is determined by two competing factors. The first factor is the momentum of the fuel jet normal to the air stream. This momentum can be expressed variously as .rho..sub.j V.sub.j.sup.2 sin .theta..sub.j where .rho..sub.j is the fuel jet density, V.sub.j is the jet velocity, and .theta..sub.j is the injection angle. The opposing factor is the drag force imposed on the fuel jet by the air approaching at velocity M . The drag force on this "body" can be computed by the normal form D=C.sub.D A q.sub.a where C.sub.D is the drag coefficient (a function of the shape of the object), A is the projected area of the fuel jet, and q.sub.a is the dynamic pressure of the air computed as q.sub.a 1/2.rho..sub.a V.sub.a.sup.2. FIG. 2, modified from Billig, F. S., Orth, R. C., Lasky, M., "A Unified Analysis of Gaseous Jet Penetration," American Institute of Aeronautics and Astronautics Journal, Vol. 9, No. 6, June 1971, pp. 1048-1058, illustrates this penetration process. For a given fuel momentum, a narrower or more streamlined fuel jet will experience a lower drag force per unit of travel distance (Y). With less deflecting force acting on the fuel jet, it will travel further into the airstream before its outward motion is arrested. It is an object of this invention to produce a narrow low drag fuel jet that can achieve these and other benefits.
The penetration and mixing of fuel jets in crossflows was extensively studied in the 1960's by Billig and others. Attempts made in this early work were met with limited success. Penetration, as used in the scramjet fuel injector art, is defined as: EQU P=Y/D.sub.j *
where
P=dimensionless penetration PA0 Y=actual penetration PA0 D.sub.j *=throat diameter of the equivalent sonic injector nozzle PA0 m.sub.j =mass flow rate (slugs/sec) PA0 .rho..sub.j =mass density (slugs/ft.sup.3) PA0 V.sub.j =flow velocity (ft/sec) PA0 A.sub.j =flow area (ft.sup.2) =.pi./4 W.sup.2 PA0 W=jet diameter (width) PA0 T=temperature of fuel jet, .degree.R PA0 R=universal gas constant PA0 P.sub.j =fuel jet pressure
Billig et al. showed that penetration is improved roughly 8% when a single supersonic (converging-diverging) as opposed to a single sonic (converging) injector is used with the same fuel flow. Billig, F. S., Orth, R. C., Lasky, M., "A Unified Analysis of Gaseous Jet Penetration," American Institute of Aeronautics and Astronautics Journal, Vol. 9, No. 6, June 1971, pp. 1048-1058. This result can be shown to be the consequence of matching the injector exit pressure to a mean back pressure surrounding the fuel jet. Matching the exit pressure produces the narrowest width jet with the highest momentum. FIGS. 3-5 illustrate fuel jets emerging into a quiescent atmosphere. FIG. 3 shows a sonic injector 10 receiving fuel 12 from a manifold. In FIG. 3, the pressure of the fuel at the exit (P.sub.e) exceeds the surrounding value (P.sub.a). Since pressure remains that could be used for additional expansion and acceleration of the fuel, this nozzle is referred to as "underexpanded" by those skilled in the art. As the jet emerges from this sonic nozzle, it has the smallest width possible for a circular jet with a given feed pressure and flow rate. Once the gases are released from the confinement of the nozzle, it is free to expand radially outward to width W to relieve the excess pressure. This produces two undesirable effects. The uncontrolled radial expansion produces less increase in the normal jet momentum than an ideal nozzle. A large radial velocity develops that causes the jet to expand beyond the value for an ideally expanded jet. The jet is now over expanded and then collapses back on itself creating a strong system of shocks including a Mach disk, which results in severe shock losses and temperature rise. This flow structure in turn results in a low density jet of significantly greater width and low momentum.
FIG. 4 illustrates a Delaval nozzle with exit pressure P.sub.a matched with the air pressure P.sub.a. Expanding the fuel to the prevailing backpressure in the Delaval nozzle produces a supersonic jet with a high velocity and nearly parallel stream lines Since the fuel pressure is matched to the surrounding atmosphere, the fuel jet can maintain its width W for significant distance beyond the injector.
FIG. 5 illustrates an "overexpanded" (P.sub.e less than P.sub.a) Delaval nozzle. In this case the fuel is accelerated to an even higher velocity and its stream lines may be near parallel at the exit but the higher surrounding air pressure P.sub.a alters the exhausting flow. Oblique shocks form at the exit that deflect the flow inward on itself As the flow converges on the centerline, additional shocks deflect the gases back parallel and raise the pressure above the surrounding value which starts an explosive re-expansion similar to the flow from the sonic injector exit As in the sonic injector (underexpanded), the overexpanded jet is lower in velocity and wider than the matched pressure jet.
An alternate confirmation of this can be obtained through use of the continuity equation, which relates the fuel mass flow rate, velocity, density and area thus: EQU m.sub.j =.rho..sub.j V.sub.j A.sub.j
where:
The continuity equation can be combined with the ideal gas law and the jet width expressed as a function of pressure and velocity thus: EQU W.sup.2 =m.sub.j RT/.pi.P.sub.j V.sub.j
where:
In under- and overexpanded flows, the shocks reduce the velocity and raise the fuel temperature, resulting in a wider fuel jet.
When extending this physics to a cross flow situation, the term "effective backpressure" P.sub.eb was defined as the average pressure varied around the jet (high on front, medium on the sides, and low on the lee or back side). The early researchers variously used 2/3 or 0.8 times the normal shock pressure to define P.sub.eb.
Another tactic to improve penetration tried by Billig et al. has been to use non-circular jets for sonic injectors to improve the aerodynamics, i.e., reduce the drag on the fuel jet by producing a narrower jet. FIG. 6 shows a normalized sketch of the fine jet structure deduced from three different shapes, taken from a Johns Hopkins Applied Physics Laboratory Seminar. Although the shape of the injector affected the shape of the fine structure of the underexpanded secondary jet, the use of the non-circular jets did not improve penetration significantly. Although this result was a surprise at the time, the following discussion illustrates one possible reason for this result. The flow of a circular jet into quiescent air was previously discussed. In that case, the backpressure on the emerging flow was uniform around the perimeter of the jet. Consequently, the jet stays circular. In a cross flow, the pressure varies according to position around the jet exit. The same effect can be expected with a non-circular jet. For the case of the oblong jet with its major axis aligned with the airflow (an apparent low drag shape), the pressure will be greatest on the front side of the injector where the air is brought to rest by the blockage produced by the jet. The pressure on the sides of the jet will be close to the free stream value. This creates a highly underexpanded condition for the flow on the sides of this jet. Under these conditions, the fuel jet can be expected to expand much more rapidly to the side making the jet more circular in shape as it moves away from the nozzle. Although Billig and others recognized the benefit of the matched pressure condition, they only applied it in an average sense. Applying the matched pressure condition locally around the perimeter of the non-circular jet will allow it to maintain it's shape further from the nozzle exit.
It was also noted in another study to improve the streamlining of the fuel jet that putting multiple sonic jets in a line parallel to the air flow, as shown in FIGS. 7a and 7b, improved penetration somewhat. In FIG. 7a the penetration of the single sonic injector 32 was compared with the penetration 34 for five sonic injectors arranged in line in the X direction. It was found that with X/D.sub.j *=7.5 resulted in approximately 20% increase in penetration compared to a single sonic jet having equal fuel flow. Penetration was measured at Billig, F. S., "Penetration and Spreading of Transverse Jets of Hydrogen in a Mach 2.72 Airstream," NASA CR-1794, March 1971.
In supersonic air flow conditions, because the flow of air is very organized it cannot go around the fuel jet easily. The air flow reacts with the obstacle and produces a shock similar to that shown in FIGS. 8a-c when a sonic nozzle is used. Shown in these figures are first and second lobes of jet fuel plume 18 and 20 separated by the Mach disk D. An exterior shock 22 develops along with a boundary layer separation shock 24. The supersonic flow at points away from the wall can tolerate a large change of angle; however, the boundary layer near the wall flows much slower (subsonic) and cannot take the increase in pressure and separates, as shown, so that a recirculation 26 takes place, forming a separation bubble. This recirculation zone creates a very high temperature region 28 on the wall of the combustor. This was also noted by Masyakin, N. E. and Polyanskii, M. N., in their paper, "The Possibility of Blowing a Gas Jet into a Supersonic Flow Without the Formation of a Three Dimensional Boundary-Layer Separation Zone", translated from Izvestiya Akadenii Nauk SSSR, Mekhanika Thadkosti i Gaza, No. 3, pp. 162-165, May-June 1979. It is a secondary objective of this invention to reduce or eliminate this hot spot.
Other fuel injectors are shown in U.S. Pat. Nos. 3,581,495; 3,699,773; 4,821,512; 4,903,480; and 4,951,463.
Although these efforts are indeed impressive, it would be advantageous if a supersonic fuel injector could produce fuel jets which penetrate further, permitting a larger number of injection sites, and mixes fuel into a supersonic air stream while reducing the loss effects noted above, so that combustors in supersonic vehicles could operate more efficiently with less combustor length than when using known injectors.